What is in this article?:

One of the largest agricultural water districts in California and a leading environmental organization have completed a joint study that shows how both interests can benefit from operating major storage reservoirs in conjunction with groundwater basins.

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Explaining results

The study explains these results as follows: Reservoirs that have dual water supply and flood control functions, like the CVP and SWP reservoirs, are typically operated under conservative rules designed to maximize water supply while avoiding flood risks. This results in relative high levels of water storage going into the winter season when the reservoirs refill, but frequent “spills” of water during the rainy season to create sufficient flood storage capacity as necessary to prevent flood damage in the downstream floodplain. These spills are a portion of the water endowment that is not controlled and therefore not appropriated for beneficial use under California water law. To capture and manage this water would require creating additional storage capacity. One way to do that without enlarging the reservoirs, or constructing additional ones, is to lower the water storage levels going into the refill period, thereby creating more reservoir capacity to capture high flows. Storage levels can be lowered by delivering additional water to meet new water supply objectives, including enhancing flows for healthier rivers and augmenting water supplies for agriculture or cities.

While the peak flood control releases do provide some environmental benefits, more focused and quantifiable benefits can be achieved by capturing and releasing them in a controlled pattern that is tailored to the needs of target species, such as salmon. In a sense, this is a strategy to use limited environmental water supplies in a more efficient manner. However, more aggressive exercise of the reservoirs to improve flow conditions for ecosystem enhancement may entail a greater risk of depleting cold water reserves needed for downstream temperature maintenance. The study is explicit about these potential environmental trade-offs.

The analysis of impacts to existing groundwater users shows that roughly 3 percent of existing domestic wells would likely be affected because these wells tend to be shallow. The report also describes measures to hold other groundwater users harmless during those years when additional groundwater extractions would be necessary to “pay back” the reservoirs. The study assumes that any such project would adhere to a "good neighbor" principle and design its mitigation plan to assure no appreciable harm to existing groundwater users. For instance, the study recommends deepening of impacted wells to and considers that cost to be a project expense. Impacts to the operability and yields of existing irrigation wells would be negligible. Modeling of the surface and groundwater systems reveals that some streamflow reductions would occur in tributary streams but would not significantly affect the primary spawning habitat for Spring‐run Chinook salmon or Central Valley steelhead trout in Butte Creek, the tributary for which sufficient data are available for conclusive analysis.